The present invention relates to a backlight device provided on the backside of a non-emissive transmission display, a method of driving a backlight, and a liquid crystal display apparatus.
As a backlight of a liquid crystal panel, cold cathode fluorescent lamp (CCFL) type backlights employing fluorescent tubes have been the mainstream. However, needs for mercury-less products have been increasing on environmental grounds. These needs have raised expectations for light emission diodes (LEDs) as a light source replacing CCFLs in recent years. In particular, a method in which each of primary color LEDs of red, green and blue is individually used and the primary colors are optically subjected to additive color mixing to thereby obtain white light, is suitable for achieving a balance among the colors. Thus, it has been intensively studied to utilize this method for television use.
If LEDs are used as a light source of a backlight, a current applied to LEDs of one color must be independent of currents to LEDs of other colors since red, green and blue LEDs have different luminous efficiency. In addition, the semiconductor composition of an LED is different dependent on the color of the LED. Therefore, the voltage applied to an element and power consumption are also different for each color. Furthermore, when using LEDs as a light source of a backlight, it is impossible to individually drive the LEDs of each color in terms of actual costs (for example, refer to Japanese Paten Laid-open No. 2001-272938).
In such a backlight using red, green and blue LEDs as a light source, light rays of these colors must be optically synthesized at a certain ratio to thereby produce white light that invariably has a certain chromaticity. Therefore, photo sensors for red, green and blue are used to sense the luminous power of the corresponding color, and currents applied to the LEDs of the respective colors are adjusted by feedback control. Thus, red, green and blue light rays are synthesized at a certain ratio to adjust the resultant light into white light having a certain chromaticity.
However, if the speed of responses to such feedback control is high, the chromaticity frequently changes and these changes are easily recognized by a user. In order to avoid the disadvantage of the frequent chromaticity changes, typically the speed of responses to the feedback control is set low. Accordingly, at the time of powering on a backlight, chromaticity adjustment by such feedback control cannot be expected.
In such a backlight employing LEDs as a light source, therefore, the initial current amount (for example, the duty ratio of pulse width modulation (PWM)) is calculated in advance for each of red, green and blue, and the LEDs of each color are driven with the initial current amount immediately after powering up. These initial current amounts are calculated according to characteristics of the backlight prior to factory shipment thereof, for example. Adequate setting of these amounts can shorten the time period from powering up until convergence of chromaticity of emitted white light on a certain chromaticity.
If the initial current amounts are not adequately set and thus the time period until convergence on a certain chromaticity is long, a phenomenon arises in which initially a pink color is displayed on the screen and thereafter the color gradually approaches white.